Infectivity is defined as the ability of a pathogen to infect and cause disease in a host organism. There are several synonyms for infectivity and these include pathogenicity, virulence, contagion, transmissibility, and communicability. Pathogenicity refers to the ability of a pathogen to cause disease whereas virulence refers to the degree of pathogenicity. Contagion refers to the spread of infection from one individual to another while transmissibility and communicability are synonyms used to describe the ease by which an infection can be transmitted from one host to another. Regardless of the synonym used, infectivity is a key aspect of understanding the spread and severity of infectious diseases.